This invention relates to semiconductor lasers, and in particular to semiconductor lasers constructed with an active layer and at least one cladding layer.
Semiconductor lasers are known in the art, however they suffer from current leakage. In the prior art devices current leakage is reduced by forming an injection blocking layer to provide a low threshold oscillation and high external quantum efficiency. An index guided structure is used to obtain stable transverse mode oscillation.
One prior art semiconductor laser 1600, having a double heterostructure, is depicted in FIG. 16 and disclosed in Electronic Letters, Vol. 21 (1985), at page 903. Viewed from a lower electrode 1601 at the bottom to top includes an N-type GaAs substrate 1602, an N-type GaAs buffer layer 1603, an N-type Al.sub.0.6 Ga.sub.0.4 As first cladding layer 1604, an undoped GaAs active layer 1605 and a P-type Al.sub.0.6 Ga.sub.0.4 As second cladding layer 1606. A rib or projecting region 1606' having inclined side faces 1610 is etched in second cladding layer 1606. A P-type GaAs contact layer 1609 is formed on the flat peak of rib 1606'. An N-type GaAs layer covers the side faces 1610 of rib 1606' which blocks injection current by forming a reverse-bias P-N junction between GaAs layer 1607 and second cladding layer 1606. Thus, the wave guide can be achieved by the imaginary part of the refractive index due to the light absorbtion of N-type GaAs layer 1607. An upper electrode 1608 is disposed across the upper surface of N-type GaAs layer 1607 and GaAs contact layer 1609.
A second prior art semiconductor laser 1400, again having a double heterostructure, is shown in cross-section in FIG. 14 and disclosed in IEEE, Journal of Quantum Electronics, Vol. QE-19 (1983), at page 1021. Laser 1400 has a layered structure of an N-type Al.sub.0.16 Ga.sub.0.84 As active layer 1403 which includes a curved portion which extends into a groove 1402a of an N-type Ga.sub.0.5 Al.sub.0.5 As cladding layer and a GaAs substrate 1404 so that the real index-guiding is realized by the difference between the refractive index of curved portion of As active layer 1403 and that outside. A semi-insulating ZnSe layer 1406 which blocks injection current is disposed on a P-type Ga.sub.0.5 Al.sub.0.5 As cladding layer 1405 disposed on active layer 1403. ZnSe layer 1406 is formed with a P-type GaAs contact layer 1408 in the region of semi-insulating layer 1406 opposite curved portion 1403a. Finally, an N-type electrode 1401 is disposed on substrate 1404 and a P-type electrode 1407 is on semi-insulating layer 1406.
When a semiconductor laser is utilized as a light source in an optical information processing apparatus, noise is generated when a part of the emiting light is returned to the cavity (hereinafter referred to as feed back induced noise), making the apparatus impractical to use. In an effort to reduce this feed back induced noise two other prior art devices, Japanese Patent Laid Open Nos. 140774/85 and 150682/85 disclose an index-guided structure at the end face of the cavity and a gain-guided structure in the center of the cavity to obtain multiple longitudinal mode oscillation. FIG. 15a is a top plan view of a laser 1506 including this structure, FIG. 15b is a cross sectional view taken along lines b--b and FIG. 15c is a sectional view taken along lines c--c. An N-type GaAs injection blocking layer 1501 is formed with a V-shaped stripe groove and layered onto a P-type GaAs substrate 1503. A wide groove 1504 is formed by etching the center of the cavity so that the wave guide forms a gain guided structure only at the center of the device. As a result the multiple longitudinal mode is obtained and the feed back induced noise can be reduced.
All of these prior art devices have proved satisfactory, however, if the injection current is blocked utilizing the reverse bias P-N junction between the N-type GaAs layer and second cladding layer 1607 as shown in FIG. 16, a junction plane is formed near active layer 1605. Furthermore, if the carrier concentration is high, breakdown voltage is low, resulting in current leakage near active layer 1605, the rise of the threshold current and the breakdown of the wall surface of rib 1610 in the high output power condition. Thus, the reliability of the apparatus decreases. Furthermore, the wave is guided by the light absorption so that the guide wave is lost and the threshold current is increased. Light absorption occurs on the side face of the wave guide, the phase of the guided wave is delayed and the wave is curved at the end face of the output side, resulting in a large astigmatism.
In the case of semiconductor laser 1400 of FIG. 14, the leakage current of the carrier which diffuses second cladding layer 1405 cannot be limited causing an increase in a threshold current, deterioration of the external quantum efficiency and an increase of the driving current resulting in deterioration of reliability.
The structure of laser 1506 in FIG. 15b is utilized for reducing the feed back induced noise. As active layer 1502 is grown on grooves of different widths, due to the differences in growing speed, the thickness of active layer 1502 in the index-guided structure is different from that in the gain-guided structure. As a result an increase of the threshold current and decrease of external quantum efficiency are caused by the loss of wave guide in the boundaries. Furthermore, the length of the gain-guided wave guide can no longer be optionally set and due to this value, the longitudinal mode becomes a single mode causing a large amount of noise against the optical feedback.
Accordingly, it is desirable to provide a semiconductor laser which overcomes the shortcomings of the prior art described above.